This invention relates generally to a motor control system and an apparatus for providing a desired three-phase voltage in the motor control system. In a particular aspect, the invention relates to such a system and apparatus adapted to operate from a higher voltage primary source while still using industry standard lower voltage components. In another particular aspect, the invention relates to such a system and apparatus adapted to operate from a lower voltage primary source that is stepped up with a corresponding step down in current. The system and apparatus can be used in operating a three-phase motor to drive a submersible pump, for example.
Submersible pumps are used, for example, in oil wells at remote locations. Three-phase electric motors are typically used to drive these pumps. Such a motor is rated for a nominal line-to-line voltage which must be provided within a specified tolerance for the motor to work. This voltage is typically provided from an electric utility through a transformer and motor controller to provide the suitable voltage and control to operate the motor as desired. A motor control system and components of the system are disclosed in U.S. Pat. No. 5,130,616 to Owen.
A specific implementation of such a system is typically made for use with a specific nominal input voltage. For the exemplary system disclosed in the Owen patent, a specific implementation is preferably energized by a nominal alternating current (a.c.) 25 kilovolt (kV) (25,000 volts) line-to-line three-phase voltage. This is a conventional utility voltage in many countries, and accordingly many of the components of the particular implementation are not designed to operate at higher voltage levels. If a higher nominal input voltage were to be accommodated, more expensive and less readily available components would preferably be needed. For example, for a nominal 25 kVac voltage, components preferably have a 150 kV BIL (basic impulse insulation level) rating as known in the industry. Components at this rating are readily available and relatively inexpensive; however, higher BIL rated components that would preferably be used at a higher voltage (e.g., 33 kVac) are not as readily available and are more expensive.
Although the aforementioned nominal 25 kVac source is standard in many countries, a higher voltage source is used in many important oil producing countries where submersible pumps are needed. For example, in several Middle Eastern countries the standard utility power source is at 33 kVac. If the invention of the aforementioned Owen patent were to be used with such a power source, a specific implementation different from the 25-kVac implementation would be needed. That is, although the invention could be used, it would have to be implemented with higher rated components than an implementation limited to a maximum nominal input voltage of 25 kVac.
To avoid having to design a different overall implementation, construct such implementation with more costly components, and maintain multiple inventories, there is the need for an improved motor control system and apparatus for such system whereby a lower voltage implementation can be more simply modified for use with a higher voltage input power source.
There are also situations where maximum current into a primary of a transformer providing the desired voltage for the motor is a significant concern. Rather than redesigning the primary to accommodate a larger current in such a case, it may be preferable for economic reasons or otherwise, to step up a lower voltage power source to a higher voltage but lower current than is drawn by the motor at the lower voltage output by the transformer. The stepped-up higher voltage would preferably be the rated nominal input voltage of the transformer necessary to obtain the desired output voltage for the motor, and the lower current would preferably not exceed the rating of the primary of the transformer at such nominal input voltage. To provide for this, there is the need for an improved motor control system and apparatus for such system whereby a current-limited implementation can be accommodated without replacing the current-limited feature.